When data are transmitted from one point to another, the data proper will be accompanied by so-called overhead information which is used for controlling and administering the entire transmission system. The overhead information in each data frame consists of a plurality of bits or bytes, each of which has a specific meaning and has a specific designation (e.g. V5, J2 or N2) in the data structure concerned.
This overhead information is to be processed in a receiver (which may be part of a regenerator or a multiplexer) receiving the transmitted data, and each of the said bits or bytes is to be interpreted separately, and the result is used in accordance with the function of the bit or byte concerned.
Typically, in a receiver a very large number of different overhead bits is to be processed in this manner, and moreover the receiver frequently receives a large number of channels in parallel, each of which contains overhead information of the same type, i.e. e.g. a J2 byte for each channel. Even the overhead bytes which do not arrive in parallel, frequently arrive so rapidly after each other that a single processor cannot manage to process one overhead byte before the next one arrives. Since, as mentioned, the overhead signals are to be processed and interpreted separately, one processor per channel and per overhead type must be available in principle. Of course, this results in a very large number of processors.
Since these processors will operate independently of each other in principle, it will moreover be very difficult to coordinate their use of a common RAM storage, as it causes problems when several processors simultaneously try to get access to the RAM storage. Consequently, each processor must have its own RAM storage, which, of course, takes up much space on the chip in which the circuit is implemented. This is a problem particularly when the circuit is implemented on a customer-designed ASIC circuit where space is exceptionally precious. The design tools used when designing such ASIC circuits will always try to locate e.g. a RAM storage as close as possible to the associated processor to avoid much and long wiring, and the many RAM storages will therefore take up much space precisely where space is most precious. Precisely the extensive wiring makes it moreover impossible to move the many RAM storages outside the circuit.
In case of a terminal multiplexer, a corresponding number of overhead information will additionally have to be generated and transmitted, thereby doubling the number of processors and RAM storages.